Aim
Oceanic islands are known for being home to drastically different communities compared to the mainland, as their isolation and limited area significantly favour clades able to travel long distances over water. On spiders, this happens mainly, although not exclusively, through ballooning whose propensity is unevenly distributed across species and requires specific conditions, possibly influencing their microhabitat distribution. However, once reaching the islands, colonisers might shift their preferences towards microhabitats with less competition. In this study, using Macaronesian spiders as models, we aim to test whether: (1) the closest mainland relatives to island endemics occupy higher vertical strata; (2) island endemics show niche shift and expansion towards lower vertical strata (lower vertical distribution and larger range compared to their closest mainland relatives); (3) active hunters show a larger vertical niche shift and expansion than web-weavers.
Location
Macaronesia (Azores, Madeira and Canary Islands).
Taxon
Spiders (Arachnida: Araneae).
Methods
We selected the closest relatives from a comprehensive phylogenetic tree encompassing Iberian and Macaronesian forest spiders. We tested our hypotheses using null models and paired Wilcox non-parametric tests followed by linear models.
Results
The mainland lineages did show higher average verticality than expected by chance. However, despite showing a significantly larger vertical range, and contrary to our expectations, island endemics display higher average verticality than their mainland lineages. Furthermore, hunters showed an increase in mean verticality, while web-weavers tended to be those decreasing, with both guilds showing no differences in the variation of vertical range separately.
Main Conclusions
Spiders living in higher layers in mainland forests have a higher probability of reaching oceanic islands; yet, not all show a preference to grab opportunities at ground level. Although this is possibly due to the presence of ground-dwelling competitors/predators or the past extinction of epigean species, the transition observed further supports the colonisation filter favouring more arboreal species.
Costa, R., Macías-Hernández, N., Rigal, F., V. Borges, P. A., & Cardoso, P. Do Island Spiders Descend From Trees? – A Tale of Island Colonisation and Niche Expansion. Journal of Biogeography, e70093. https://doi.org/10.1111/jbi.70093
